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Melanocytes are cells located in the bottom layer of the skin's epidermis. With a process called melanogenesis, they produce melanin, a pigment in the skin, eyes, and hair. In Caucasians, and some Asians, melanocytes are only triggered by ultraviolet rays, thus requiring sun exposure in order to tan. Amongst darker peoples, the melanocytes constantly produce melanin and this can also be enhanced by sun exposure.
The typical density of melanocytes is between 1000 and 2000 cells per square mm of skin, comprising between 5-10% of cells in the skin. Although the size may vary, the typical size of a melanocyte is 7 micrometers in length. The difference between fair people and dark people is not the number of melanocytes, but how active the melanocytes are.
Embryologically, melanocytes come from the neural crest. This is completely different to the surrounding skin cells (keratinocytes). This fact also means that a cancer of a melanocyte, a melanoma, will spread (metastasize) very easily. For this reason melanomas are often fatal, and when being removed a lot of surrounding tissue needs to be taken as well.
Melanin is initiated by either MSH (melanocyte stimulating hormone) ATCH (another hormone) or ultraviolet light. Once it is made then it is stored in “the warehouses”. These are the arm like structures called dendrites. From there they are shipped to the keratinocyte.
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